The present invention relates to a recording density correction apparatus in a printer for performing thermal transfer recording, thermo-sensitive recording, or the like.
A thermal transfer recording system has been put into practical use as a recording apparatus such as a printer, facsimile equipment or the like, and the system has been widely manufactured. This recording system has only a simple process in which ink is fused or sublimated by heat generated from heating elements constituting a thermal head and the fused ink is made to adhere on recording paper. In this recording system, however, irregularity is cased in size or density of recorded dots to thereby cause unevenness in recording density in the whole of a recorded picture because of variations in heating temperature due to irregularity in resistance value among the heating elements of the thermal head or the like.
In a conventional system, therefore, in order to prevent the unevenness from occurring in recording density, the respective resistance values of the heating elements are detected in advance and stored in a storage circuit so that the energy to be applied to the heating elements in recording is controlled in accordance with the stored resistance values.
Referring to FIG. 5, the configuration of the conventional example will be described. FIG. 5 is a block diagram showing the conventional recording density correction apparatus in a printer as described, for example, in "TECHNIQUE FOR REALIZING HIGH PICTURE QUALITY OF HIGH QUALITY VIDEO COPY", in the Collection of Paper of the Third Non-Impact Printing Technique Symposium, 1986, pp. 37-40.
In FIG. 5, the conventional recording density correction apparatus in a printer is constituted by a counter connected to a clock signal generation circuit (not shown), an EPROM 3 connected to an arithmetic unit 1 such as a personal computer, a minicomputer or the like and the counter 2, and an EPROM 4 connected to the EPROM 3.
Next, referring to FIG. 6, the operation of the foregoing conventional example will be described.
FIG. 6 is an explanatory diagram showing correction gradation level data stored in the EPROM 4 of the conventional recording density correction apparatus in a printer.
First, the arithmetic unit 1 measures the respective resistance values of heating elements constituting a thermal head in advance, divides the heating elements into groups in accordance with the resistance values, determines respective correction factors for the heating elements, and writes class numbers of the correction factors in the EPROM 3 so that the correction factors corresponding to the heating elements may be selected. Alternatively, the arithmetic unit 1 may optically measure unevenness in recording density of a recorded picture so that the heating elements are divided into groups in accordance with values of the thus obtained correction information.
The counter 2 is made to operate in response to a clock signal from the clock generation circuit so that address signals A1 corresponding to the respective heating elements are supplied to the EPROM 3.
The EPROM 3 supplies the EPROM 4 with class numbers (1-16) corresponding to the respective address signals A1, that is, corresponding to the respective heating elements.
The EPROM 4 corrects the input gradation levels D of the drive signals for the thermal head on the basis of the class numbers, that is, the address signals A2. That is, respective correction graduation level data D. shown in FIG. 6 are supplied to the heating elements.
In the foregoing conventional recording density correction apparatus of a printer, however, there has been a problem in that when a recordable density gradation scale is set, for example, to 1/64, the drive signals applied to the heating elements can be corrected only by gradation on 1/64 even if grouping is performed in accordance with a correction factor having correction accuracy of 1/128, and therefore a difference in density between recorded dots adjacent to each other cannot be finely corrected.